Word for stealing an item then returning it without the owner's knowledge?

Consider, Indian taking

Indian taker

Informal. Offensive A person who steals your property but returns the stolen property to you at a later date.

The car thief stole my car but was nice enough to return it the next day. What an Indian taker piece of shit.

Urban Dictionary

Indian giver

One who takes or demands back one's gift to another, as in Jimmy wanted to take back Dan's birthday present, but Mom said that would make him an Indian giver. This term, now considered offensive, originally alluded to the Native American practice of expecting a gift in return for one that is given. [; early 1800s]

The American Heritage® Idioms Dictionary

Here's an expanded version of my earlier comment, as suggested. The dictionary definitions are straightforward and some have been provided by others. I'll just link to them as necessary rather than print them in full here.

There are two actions here: taking the item, and then bringing it back. Based on the title, what happens to the item in between those two actions is provided as background knowledge but outside the scope of this question.

The first action is theft, and the question's clarifying edit makes it explicit that it is stealing.

Since the item is returned, it can also be said to have an element of borrowing, except that borrowing casts it in a more innocent light than warranted.

The owner might charitably consider the whole activity to be unauthorised borrowing. To someone who knows the facts as presented in the clarifying edit, it is the theft and return of the goods.

Stealing and then returning it is of course stealing. If the thief has a bad conscience and returns the goods, or is afraid that he or she may be found out and returns the goods, it is still stealing. I assume that you meant "Taking something away without permission or knowledge of the owner and later returning it".

This will have some legal consequences, which will be depending on where exactly you live. It might be legally theft, it might not. Some places have special laws for joyriders who take a car, drive around, and will always claim they intended to return it, so this is illegal and punished the same as theft, even if it isn't.

But we are talking about language, not about laws. You can call it whatever you like even if it doesn't match the laws of your country. You can say "To me, it's stealing, I don't care what the law says", if that's what you think. In written language, you can call it "borrowing" in quotes, including the quotes, because it is similar to borrowing but you are not convinced that it is the same.

The "diary" case I would call a severe violation of the privacy of the diary owner. Compare it to another diary owner who has bought a new diary for 2017 already and that empty diary gets stolen and not returned; that diary owner will probably be a lot happier than the one whose full diary with personal notes was taken away and returned.

To avoid confusion, you can say "Someone took X away without my permission and returned it three days later". Then everyone knows what you mean. As long as you don't know the situation, it's hard to say more. I have two neighbours, one is quite Ok, but I hate the other one. If A asked for my lawn mower, they would get it. B wouldn't. Now my lawnmower disappears for a day and returns. It makes a difference to me whether A or B took it.